The present invention relates to an assembly for focusing and coupling the radiation produced by a semiconductor laser diode) into an optical fiber, in particular a multimode optical fiber, with a radiation-focusing element in the form of a cylinder lens being disposed in the area between a radiating surface of the laser and a light entrance side of the optical fiber, the cylinder lens being oriented substantially parallel to the multimode direction of the radiating surface of the semiconductor laser. The invention also relates to a laser module having a plurality of assemblies of the type defined above.
Finally the invention relates to an assembly for focusing and coupling the emissions produced by a semiconductor laser array (laser diode array) or a two-dimensional semiconductor laser structure (laser diode stack) into a corresponding number of individual optical fibers, in particular multimode optical fibers, with an optical system for focusing radiation being provided in the area between the radiating surface of the laser diode array or stack and the individual light entrance sides of the optical fibers.
The source xe2x80x9cApplied Opticsxe2x80x9d, Vol. 16, no. 7, Jul. 1977, pp. 1966-1970, discloses an assembly having the features described in paragraph 1 of the xe2x80x9cTechnical field of the inventionxe2x80x9d above. This source (see in FIG. 5 on p. 1968) describes a specially designed carrier element for coupling a semiconductor laser, e.g. a GaAs diode, and an optical fiber having a circular cross section, said element simultaneously carrying the cylinder lens for coupling the laser radiation produced by the GaAs diode into the subsequent optical fiber. This substantially cylindrical carrier element is provided on the top with a given number of V-groove recesses which are preshaped by mechanical, i.e. metal-removing, processing of the carrier element material, e.g. copper, in accordance with the intended coupling between laser, cylinder lens and optical fiber and in order to guarantee the necessary optical adjustment of these elements to one other. Due to this design of the carrier element the optical elements subsequent to the GaAs diode, i.e. the cylinder lens and the optical fiber, can be inserted into these above-mentioned recesses, the mutual adjustment of these elements being dependent on the precision with which the corresponding recesses have been machined in the carrier element. Very low tolerances are of course permissible here to allow for the necessary ultimate adjustment.
The mechanical requirements for the tolerances of such a carrier element, and in particular the required positioning accuracy for the GaAs diode relative to the subsequent optical elements, are thus extremely high since particularly this laser diode must be applied and fastened to the surface of the carrier element, for example by gluing or soldering. This means that the production and processing of such a carrier element is very elaborate to permit the necessarily very low tolerances to be met.
This known assembly is intended to focus as much as possible of the radiation emitted by one semiconductor diode into the core of the optical fiber, e.g. a multimode fiber with a circular cross section. There is no intention here to provide a laser module with the greatest possible power and radiation quality.
Further, xe2x80x9cApplied Opticsxe2x80x9d, Vol. 17, No. 3, 1978, p. 479 ff., discloses an assembly wherein each individual laser of a monolithic laser diode array is coupled by means of a common cylinder lens to a great number of individual optical fibers. A carrier element in the form of a silicon substrate is likewise used here, a given number of V-shaped recesses being preshaped in this silicon substrate in accordance with the intended arrangement of cylinder lens and optical fibers for coupling between a laser diode and an optical fiber as in the known coupling assembly explained above.
Extremely low process tolerances are also permissible in this case. In addition, the direct contact between cylinder lens and laser diode array can drastically change the properties of the individual laser diodes, i.e. the laser diodes can very easily be mechanically destroyed when the cylinder lens is inserted into the V-shaped groove provided in the silicon substrate. In this latter assembly the manufacturing expenditure is therefore likewise considerable.
In view of the prior art described above the present invention is therefore based on the problem of providing an improved assembly for focusing and coupling the radiation produced by a semiconductor laser into an optical fiber, in particular a multimode optical fiber, primarily with the goal of realizing a highly efficient coupling between a multimode high-power laser diode and a multimode optical fiber. The invention is also based on the problem of providing a laser module, in particular a pumping module, with much greater power and maximum radiation quality as compared with known laser diode modules.
Finally there is also the requirement that assemblies utilizing the above-described type of coupling should be able to be produced in a much simpler and less expensive way compared with the prior art explained above, while ensuring all requirements for the precision of adjustment. The first mentioned problem is solved according to the invention by adapting a cylinder lens of the above-mentioned type to have a substantially equal to the width of a beam exit window defining the radiating surface of the semiconductor laser and whose diameter is substantially on the order of magnitude of the core diameter of the optical fiber, and preferably smaller than this core diameter, and by directly gluing the lens to, fusing the lens with or melting the lens onto the light entrance side of the optical fiber extending substantially at right angles to the orientation of the cylinder lens.
Further, the present invention makes it possible to obtain a laser module with high power and high radiation quality while having a small numerical aperture. A given number of individual assemblies designed according to the present invention are united into a module, the corresponding individual optical fibers being bunched such that the free ends, i.e. the light exit sides, of all optical fibers are disposed in a configuration that can be selected or predetermined at will.
A laser module of the type characterized above can be used in preferred fashion as a pumping laser for systems with pumped solid state lasers, the total laser radiation obtained on the light exit sides of the bunched optical fibers serving as the pumping energy for the subsequent pumped laser.
Such optical pumping (longitudinal and transversal) can be performed for example with or without frequency multiplication, for example for pumping a neodymium-YAG laser. It is also possible to use a laser module of the type characterized above as a laser particularly for medical applications or for materials processing. In such cases, the total laser radiation obtained on the light exit sides of the bunched optical fibers is utilized as the energy for the particular medical treatment process or the particular materials processing operation.
Furthermore the light exit sides of the bunched optical fibers can preferably be combined in a matrix-shaped assembly or a linear assembly. According to yet another feature of the invention any desired symbols can be applied in matrix form to an object as xe2x80x9claser markingxe2x80x9d by a preferably selective and individual drive of the individual diode lasers and imaging on the object. In a corresponding way, machining places can be preselected in matrix form strictly by the electric drive of the individual diode lasers, which provides special advantages in particular for micromachining, e.g. when soldering, welding or drilling.
A further possibility of designing a laser module within the scope of the present invention is to couple and combine the radiating surfaces of laser diode arrays or of two-dimensional structures, so-called laser diode stacks, into corresponding optical fibers, instead of using individual assemblies of the type explained above each having individual laser diodes.
In an assembly for focusing and coupling the emissions produced by a semiconductor laser array or a two-dimensional semiconductor laser structure into a corresponding number of individual optical fibers according to the features of yet another embodiment of the invention, the optical system for focusing radiation is formed by a cylinder lens dimensioned in accordance with the number and arrangement of the individual optical fibers and oriented substantially parallel to the multimode directions of radiation of each laser diode.
The length of the lens is adapted substantially to the total width of the beam exit windows defining the radiating surface of the laser diodes and its diameter is substantially on the order of magnitude of and preferably smaller than the core diameter of the particular individual optical fiber. The cylinder lens is directly glued to, fused with or melted onto all associated light entrance sides of the optical fibers extending substantially perpendicular to the orientation of the cylinder lens.
According to further advantageous embodiments of the assembly, if semiconductor laser arrays are used, the individual cylinder lens is preferably formed as a glass fiber lens. The same applies if two-dimensional semiconductor laser stacks are used. The additional advantage of such semiconductor laser arrays or stacks is their high-precision production and the related low positional tolerance of the radiating surfaces of the corresponding semiconductor lasers or laser diodes.
A relatively small air gap may preferably be provided between the laser diode array or stack and the assembly of the cylinder lens of the optical system for focusing radiation, regarded in the directions of radiation. This avoids mutual mechanical influence between the cylinder lens and the accordingly associated laser diodes, with the advantage that neither the properties of these laser diodes are changed nor is there a danger of the laser diodes being mechanically destroyed.
The invention furthermore includes the very advantageous possibility of coupling the laser diode array or stack to the associated number of individual optical fiber-cylinder lens units by providing a holding means common to all these units. Using standard techniques (etching, CNC milling and the like) one can produce structures for such holding means in a very simple and inexpensive way with a tolerance in the micrometer range sufficient for the desired coupling of the laser diode arrays or stacks to the corresponding number of optical fibers.
If the cylinder lens is glued on for example with the aid of an epoxy adhesive, one additionally obtains the special advantage that this adhesive centers the cylinder lens or fiber lens itself on the light entrance sides of the individual optical fibers. A further advantage of the direct gluing method is obtained by using an epoxy adhesive that is very thin and has very low absorption at the laser wavelength of approximately 810 nm emitted by the laser diodes for example. Excess adhesive tends to increase the coupling efficiency even further.
On the basis of first experiences it can be said that a coupling efficiency of about 70% is typically obtainable, but a coupling efficiency up to almost 100% is also theoretically possible.
If a semiconductor laser array (laser diode array) or a two-dimensional semiconductor laser structure (laser diode stack) is provided in an assembly according to the present invention this assembly can also be used as a pumping laser for systems with pumped solid state lasers, the total laser radiation available on the light exit sides of the preferably bunched optical fibers serving as the pumping energy for a subsequent pumped laser.
It is especially advantageous to bunch all individual optical fibers of the corresponding semiconductor laser array or the corresponding two-dimensional semiconductor laser stack in such a way that the free ends, i.e. the light exit sides, of these optical fibers are disposed in configurations that can be selected or predetermined at will.
For example, a configuration can be provided such that the free ends (light exit sides) of the optical fibers are combined in a geometrically closest packing.
Furthermore, a configuration can be selected such that the free ends (light exit sides) of all optical fibers form a substantially rectangular matrix. On the other hand, a configuration is also possible such that the free ends (light exit sides) of all optical fibers form a line.
Finally, it is also possible to drive the individual laser diodes selectively and individually so that the radiation emerges from a corresponding selection of light exit sides of the optical fibers, allowing the production of beam exit patterns that can be selected or predetermined at will. Using such an assembly one can for example mark an object with a pattern representing whole letters, numbers or other symbols in matrix form or representing parts of letters, numbers or other symbols so that several matching parts result side by side in whole letters, numbers or other symbols in matrix form.
The ends of the combined fibers can be imaged on a given place on an object (the image plane) either directly, if the distance to the image plane is small relative to the total diameter of the fiber bundle, or with the aid of an optical system, so that the laser light of the luminous fiber ends produces on the surface of this object a change which causes the desired marking of the object.
There are a number of preferred possibilities of application for the inventively designed assembly. For example, one such application is to use the assembly as a pumping laser for systems with pumped solid state lasers, the laser radiation obtained on a light exit side of the optical fiber serving as the pumping energy for the subsequent pumped laser.
The possibility of use described above also exists, for example, when, a given number of assemblies according to any of the embodiments are united into a module, the corresponding optical fibers of these individual assemblies being combined such that the free ends (light exit sides) of all optical fibers are disposed in a configuration that can be selected or predetermined at will. Furthermore, an inventively designed assembly or a corresponding laser module can be preferably used for tissue removal, coagulation, heat treatment, stimulation or other optical treatments in the medical field, or for materials processing, the laser radiation obtained on the light exit side of the assembly or the laser module serving to produce the above-mentioned effects.
If an inventive assembly or an inventive laser module is used for flexible materials processing or for tissue treatment it is also possible for the ends of the combined fibers to be imaged on a given place on a workpiece or tissue (i.e. the image plane) either directly, if the distance to the image plane is small relative to the total diameter of the fiber bundle, or with the aid of an optical system, the desired materials processing or tissue treatment then occurring at this place.